1. Field of the Invention
This invention relates to active filters, to methods of calibration, and to corresponding software and integrated circuits.
2. Discussion of the Related Art
It is known to provide active filters using op-amps (operational amplifiers) with feedback. In general such a filter is illustrated in FIG. 1a in a negative feedback arrangement. This shows an op-amp, an input resistor R2, and a feedback resistor R1 coupled from the op-amp output to the op-amp negative input. Typically R1=R2 and both have a resistance value R, but other arrangements are possible. A capacitor C1 is coupled in parallel with the feedback resistor. It is known to use OTAs (Operational transconductance amplifiers) in such filters. The OTA is a voltage-controlled current source whereas a conventional op-amp is a voltage-controlled voltage source. The OTA is similar to a standard op-amp in that it has a high impedance differential input stage and that it may be used with negative feedback. An additional input for a current (Iabc) can be provided to control the amplifier's transconductance. The OTA has both inverting and noninverting inputs; power supply lines and a single output as well as the additional biasing input, Iabc. Optionally, a further biasing input Ibias—see FIG. 1b—can be provided. As depicted in the figure, the transconductance of the amplifier is controlled by the input current Iabc (“amplifier bias current”). The amplifier's transconductance is directly proportional to this current. Optionally, as also depicted in the figure, the internal design of the OTA includes diodes whereby the anodes of the diodes are attached together and the cathode of one is attached to the non inverting input and the cathode of the other to the inverting input. The diodes are biased at the anodes by a current (Ibias) that is injected into the Ibias terminal. These biasing currents make two substantial improvements to the OTA. Firstly, when used with input resistors, the diodes distort the differential input voltage to offset a significant amount of input stage non linearity at higher differential input voltages. Secondly, the action of the biased diodes offsets much of the temperature sensitivity of the OTA's transconductance.
An OTA can enable filter circuits with voltage-variable control of the midband gain of the circuit, or external bias setting can be used to control the center frequency, or 3-dB frequency, or shape of response, in a filter. For RC filters, it is known that there will be variations in the values due to manufacturing tolerances. Since the pole frequency is mainly determined by the product R×C of such a filter, this product needs to be calibrated. Hence, it is known to reduce the amount of variation by providing an RC calibration circuit. However not all effects are compensated so that it is known to provide a margin of error in the system having the filter. For example, the gain bandwidth of the OTA has an effect on the pole frequency and so it is known to provide a margin of safety e.g. to allow for a variation in the gain bandwidth of the OTA between devices. Providing a margin of safety can result in extra semiconductor/chip area and power consumption. For example, the gain bandwidth of the op-amp can be made so large that there is little effect on the filter pole (so that variations do not matter) but this results in higher current consumption.